Study of creep cavitation in a stainless steel weldment

A study of creep cavities near reheat cracking in AISI Type 316H austenitic stainless steel headers, removed from long-time high temperature operation in nuclear power plants, is reported. It is shown how application of scanning electron microscopy (SEM), cryogenic fractography and small angle neutron scattering (SANS) can be applied, in a complementary way, to observe and quantify creep cavitation damage. Creep cavities in the vicinity of the crack are found to be mainly surrounding inter-granular carbides. Trends in the size and area fraction of creep cavities relative to the crack path are quantified based on an optimised metallographic and image analysis procedure. Brittle fracture, with clear facets, of material remote from crack is achieved using a cryogenic procedure, and is compared with fractured facets of material within 8 mm of the crack which show evidence of large numbers of cavities. The SANS technique is found to be a very suitable method of quantifying creep cavitation averaged over a larger gauge volume. The distribution of cavities within the size range of 100 to 400 nm is quantified using SANS from positions normal to the crack line. It is shown that the cavity size distribution peaks at about 200 nm and this correlates closely with the quantitative SEM observations.